Servo systems in optical data recording devices such as optical tape drives utilize tracking error signals, detected from the optical media via an optical pickup unit (OPU) device, to accurately record and then retrieve data on the optical media.
FIGS. 1 and 2 illustrate a portion of a typical optical recording medium. FIG. 1A is a top view while FIG. 1B is a side view. Optical recording media 10 includes a nanostructure surface relief pattern embossed on the surface of the optical medium. The nanostructure includes lands 12 and grooves 14 embossed in the Z direction (i.e., perpendicular to the face of optical recording medium 10) thereon in a preformatting process. These surface relief patterns are used to generate the tracking signals used by a servo system to track the position of an optical head reading or writing to the medium. An optical drive OPU with the aid electronic signal processing generates a tracking error signal (TES) from the detected patterns. In order to establish an addressing capability for these recording tracks, the edges of these embossed lands 12 and grooves 14 relief patterns are structurally modulated in the horizontal direction parallel to the face of optical recording medium 10 (e.g., Y axes to track X axes) with sinusoidal patterns 16 (i.e., wobbles) which contain individual track address codes. FIG. 1A also depicts recording marks 18 encoded thereon.
A technique referred to as “Radial Push Pull” Tracking signal generation (also referred to as “Main Push Pull” (MPP), has been conventionally used to generate the Tracking Error Signal (TES) for the rewritable optical recording media preformatted with “land” and “groove” track geometries as set forth above. This scheme generates a reference tracking signal based on the geometries of land and grooved tracks on the media and detectable by a main quad photodetector (QPD) of the OPU. FIG. 3 provides a schematic illustration of a typical signal processing scheme for the TES signal generated by the QPD. Signal processing system 20 includes recording/reading head 21. Recording/reading head 21 includes quad photodetector 22 which includes individual photodetectors 24, 26, 28, and 30. Signals 32, 34, 36, 38 from photodetectors 24, 26, 28, 30 are amplified by amplifiers 42, 44, 46, 48 to provide signals 52, 54, 56, 58. Signals 52, 54 are provided to adder 60 which outputs summed signal 62. Signals 56, 58 are provided to adder 64 which outputs summed signal 66. Summed signal 62 and summed signal 66 are inputted into subtractor circuit 70 with outputs difference signal 72 which is further processed to provide TES signal 78 and wobble signal 80. For example, low pass filter 82 receives difference signal 72 as an input and outputs TES signal 78, while band pass filter 84 receives difference signal 72 and outputs wobble signal 80. The high frequency wobble signal includes, among other information, the key data track ID and Address codes. Moreover, TES signal 78 and wobble signal 80 are used by recording/reading head servo system 86 to provide positioning information regarding the position of head 21. In particular, digital servo systems control the dynamic operation of the OPUs by using wobble signal information to place the OPU on the correct desired data track.
As depicted in FIG. 4, the Radial Push Pull method of TES derivation generates a quantized sinusoidal signal as the OPU 22 moves across multiple data track on the media 10 along direction d1 while the medium is moving along direction dtape. It is a well-known shortcoming that directional information is not provided by this method because of the quantized sinusoidal nature of the signal as depicted in FIG. 5. FIG. 5 demonstrates that movement first along direction d1 and then along direction d2 produces the same TES signal as movement only along direction d1. This lack of direction information has a severe impact on the robust control of the tracking servo system especially during cross track OPU motion. It is significant that the TES signals from both FIGS. 4 and 5 do not show any difference as OPU motion changes direction.
Accordingly, there is a need for improved methods and apparatuses for detecting the direction of OPU motion.